Lithium ion secondary batteries, which are nonaqueous electrolyte-type energy devices that have high energy densities, are widely used as a power source for portable information terminals such as notebook personal computers, portable phones, and PDAs.
The negative electrode active material used in these lithium ion secondary batteries (referred to hereafter simply as lithium batteries) is a carbon material that has a multilayer structure that has the ability to intercalate the lithium ion between layers (formation of a lithium interlayer compound) and to discharge the lithium ion. A lithium-containing complex metal oxide is primarily used as the positive electrode active material. The electrodes of lithium batteries are fabricated by preparing a slurry by mixing and kneading these active materials with a binder resin composition and a solvent (for example, N-methyl-2-pyrrolidone or water); coating this slurry, using, for example, a transfer roll, on one or both surfaces of a metal foil that acts as a current collector; forming a composite layer by drying off the solvent; and thereafter compression molding with, for example, a roll press.
Polyvinylidene fluoride (PVDF) is frequently used for the aforementioned binder resin composition. However, PVDF exhibits a poor adhesiveness for the negative electrode current collector (copper foil), and, when a negative electrode is fabricated using PVDF, it therefore becomes necessary to blend large amounts of PVDF relative to the negative electrode active material in order to secure adhesion at the interface between the composite layer and current collector, which impedes the effort to raise the capacity of lithium batteries.
Moreover, PVDF is not always sufficiently resistant to swelling by the liquid electrolyte (liquid that mediates lithium ion transfer between the positive and negative electrodes during charge/discharge) used in lithium batteries, and when as a consequence the PVDF in the composite layer is swollen by the liquid electrolyte, the composite layer/current collector interface is loosened up, as is contact by the active material with itself in the composite layer. This leads to a gradual disruption of the conductive network in the electrode and has been a factor in causing a timewise decline in capacity during the repetitive charge-discharge cycling of lithium batteries.
As a solution for these problems, Japanese Patent Application Laid-open No. 2003-132893 discloses a modified poly(meth)acrylonitrile-type binder resin obtained by copolymerization with a short-chain monomer such as C2-4 1-olefin and/or alkyl(meth)acrylate in which the alkyl group has no more than 3 carbons. Japanese Patent Application Laid-open No. 2003-132893 also discloses a binder resin obtained by blending this modified poly(meth)acrylonitrile-type binder resin with, for example, a rubbery component having a glass-transition temperature of −80° C. to 0° C. In addition, the use of a binary copolymer of acrylonitrile and short-chain methyl methacrylate as the binder resin is also disclosed in Journal of Power Sources 109 (2002) 422-426.
However, poly(meth)acrylonitrile is inherently a polymer with a rigid molecular structure, and the copolymers with short-chain monomer as described in the aforementioned documents create problems with regard to the softness and flexibility of the resulting electrode, even when a rubbery component is blended therein. This has created the risk of the generation of defects, for example, cracking in the composite layer, during the fabrication of lithium battery electrodes, for example, during roll press molding or during the step in which the positive electrode and negative electrode are wound with an interposed separator into a coil.